Field of the Invention
The invention relates to a method for calculating the required bit rate of a number of communication connections that can be statistically multiplexed. The invention further relates to a method for controlling an acceptance or refusal of a new connection on an ATM communication device with a capacity R.sub.max that is already handling M connections that can be statistically multiplexed.
In the asynchronous transfer mode (ATM), data are transmitted independently of the information represented by them (voice communication, data communication, multimedia) in cells of 5 bytes (48 bytes of useful data and 5 bytes of control data). The network resources of a communication device, such as for example a multiplexer, a line or a switching matrix, are in this case shared by connections with different grade-of-service and bit-rate requirements. At the same time, it must be ensured by a so-called traffic control that, in spite of the joint transmission of data cells of various origin, various bit rates and various bit rate statistics, the required grade of transmission performance of the ATM layer is ensured. In particular, it must be ensured that the probability of cell loss is very low, for example less than 10.sup.-10, and that transmission time variations of the cells do not exceed a certain value.
In an ATM network, various types of connection are possible, distinguished by their bit rate statistics. A connection with a constant bit rate or deterministic bit rate has a fixed transmission bit rate (cell rate) over the entire duration of the connection. This bit rate must be constantly provided by the network. This type of connection is particularly suitable for real-time applications, such as voice communication for example, in which strict requirements are imposed on the cell delay variations and which have a virtually constant transmission rate.
A further type of connection is the available bit rate, the data being transmitted according to the network capacity available at a given time. This type of connection is not suitable for real-time applications, but for example as a low-cost data transmission, such as e-mail for example.
With the statistical bit rate type of connection, the data to be transmitted are transmitted on a virtual connection with a transmission rate varying over time. Examples of such a type of connection are video connections in which the video signals are encoded with a variable bit rate and voice communication with pause suppression and certain data transmission services. Connections with a statistical bit rate, in which the average bit rate is significantly below the maximum bit rate are suitable for statistical multiplexing. In this case, many connections are carried with a statistical bit rate over a common line or a common switching matrix, it not being necessary for each individual connection to reserve the maximum bit rate, since many uncorrelated connections with a low average bit rate in comparison with the maximum bit rate share the available transmission capacity on average. It is thus possible to "overbook" the line to a certain extent. The network infrastructure can be better utilized overall in this way.
To enable the network operator to provide adequate capacity for a number of communication connections with a statistical bit rate that are independent of one another, the maintenance of certain traffic parameters must be ensured by technical precautionary measures at the terminal devices or the like in a so-called traffic agreement. The traffic agreement regulates among other things the maximum bit rate (or peak cell rate, PCR) and the average bit rate or corresponding average sustainable cell rate (SCR). The peak cell rate PCR in this case gives the maximum number of ATM cells per unit of time taken up by the connection and the average sustainable cell rate SCR gives the average number of ATM cells per unit of time permissible over a prolonged period of time.
The problem with the connection acceptance control of connections which can be statistically multiplexed, i.e. connections with a statistical bit rate for which the ratio of the maximum bit rate to the average bit rate is above a certain value, is that it is necessary to avoid cell losses, which may occur due to the simultaneous transmission of many connections at a high bit rate. It is also necessary to make it possible for the ATM connection or the ATM communication device to be utilized to the greatest possible extent. Various connection acceptance methods of this kind are known.
One possibility is to reserve the peak cell rate PCR for each communication connection. Consequently, cell losses caused by overloading of the communication connection cannot occur, but the advantages of statistical multiplexing, i.e. the better utilization of the communication device by connections with a varying cell rate that are independent of one another cannot be exploited.
If, on the other hand, only the average sustainable cell rate SCR is reserved for each connection, intolerable cell losses occur even when there are small variations in the overall cell rate. Only when there is a very high number of connections that are independent of one another does the capacity required for transmission of the connections without any cell loss come close to the sum of the average sustainable cell rates of the individual connections.
One known method for controlling the acceptance of connections which can be statistically multiplexed is the so-called sigma rule, which is described in European Patent EP 0 433 699 B1 and in a reference by Rathgeb, Wallmeier titled "ATM-Infrastructur fur die Hochleistungskommunikation" [ATM Infrastructure For High-Performance Communication], pages 148 to 150. In this method, an additional connection is still accepted along with a number M of already existing connections if an upper estimate of the transmission bit rate necessary for the transmission of the M+1 connections is less than or equal to the maximum bit rate R.sub.max of the communication device.
The estimate of the required capacity in the case of the sigma rule is given by the addition of the sum S.sub.M+1 =.SIGMA.SCR.sub.i of the average sustainable cell rates of the M+1 connections which can be statistically multiplexed to a factor Q(R)*V.sub.M+1, where Q(R) is a quantile function which indicates the statistical behavior of the connections in dependence on the required bit rate, and V is an estimate of the variance of the bit rates of the M+1 connections.
If the capacity of a transmission device is shared with other types of traffic, for example traffic at an unspecified bit rate or available bit rate, the capacity R available for the traffic to be multiplexed is no longer known.
Until now, the sigma rule has been extended to cope with this problem and the capacity of the M already reserved connections used as the decision parameter for the acceptance of the M+1th connection. This capacity is iteratively increased when setting up further connections, to be precise by the average sustainable cell rate of the connection to be added if the sigma rule will accept the connection for this capacity, otherwise by the peak cell rate. By this procedure, the capacity determined is dependent on the setting-up sequence.